The theory of stress corrosion for slow crack propagation is reviewed in the light of classical Griffith theory of fracture. Experimental data for stress corrosion cracking for glasses, ceramics, and metals are reviewed. We suggest that stress corrosion cracking plays an important role in the intrusion of magmas and in the transport of magmas upward through the lithosphere. It is shown that the effect of decreasing temperature (at progressively shallower levels along the geotherm) would be to decrease the crack velocity by several orders of magnitude if other factors were equal. We also propose that stress corrosion may be an important process in time-dependent earthquake phenomena such as premonitory behavior and earthquake aftershocks. We suggest that slow cracking in the earth is not seismically detectable but may nevertheless precede the terminal (catastrophic) phase of the fracture that is discerned as an earthquake. The seismically quiet periods before some earthquakes and the seismically quiet regions beneath some volcanoes may in fact be regimes of slow crack propagation. Slow crack propagation in a lithospheric plate may provide access routes for magmas which give rise to prominent linear volcanic chains.

Stress corrosion theory of crack propagation with applications to geophysics

Water and electrolytes cause significant changes in brittle fracture strength and subcritical fracture propagation velocities in quartz and quartz rocks. The changes may be caused, in part, by changes in surface free energy. Experimental fracture surface energies of quartz range from about 400 mJ m/sup -2/ to about 11.5 J m/sup -2/. Thermodynamic surface free energies are likely to be lower than fracture surface energies owing to dissipative energy losses and failure to achieve equilibrium surface structure. Thermodynamic surface free energies are sensitive to environmental composition. Reaction of water vapor with pristine fracture surfaces reduces surface energy by hydroxylation, but the extent of reduction is not known. Adsorption of water vapor on the hydroxylated surface and immmersion in liquid water reduce surface energy by 75--230 and 72 mJ m/sup -2/ respectively. In an electrolyte, the surface energy by 75--230 and 72 mJ m/sup -2/ respectively. In an electrolyte, the surface free energy is maximum at the point of zero charge, where adsorption of ionic solutes is least. Adsorption of hydrogen ion, hydroxide ion, and electrolytes reduce surface energy as concentration increases, by tens of mJ m/sup -2/. All of these surface energy changes are qualitatively consistent with changes in fracturemore » behavior caused by the same variables.« less

Surface and interfacial free energies of quartz

R estrictions on the quasi-static extension, or healing, of Griffith cracks are developed in the framework of irreversible thermodynamics. It is emphasized that thermodynamics requires that (G − 2γ)ι ⩾ 0, where ι is crack speed, G the Irwin energy release rate, and 2γ the work of reversible separation of the surfaces to be fractured. Implications for ‘lattice trapping’ models of cracks and for thermally-activated crack motion are discussed, as are the effects on crack growth and healing of a surface-reactive environment, in which case γ must be given a definition appropriate to adsorption-altered surface properties.

Thermodynamics of the quasi-static growth of Griffith cracks

The acoustic emission of Westerly granite sujected to temperature changes up to 120 /sup 0/ at various heating rates from 0.4 to 12.5 /sup 0/C/min was studied. A threshold temperature of 60 to 70/sup 0/ c appeared to exist for this range of heating rate, above which A.E. began to occur profusely with increasing temperature. Above the threshold temperature, the rate of A.E. depended strongly on the rate of heating. However, a thermal' Kaiser' effect appeared to exist such that in cyclic heating and at temperatures below the maximum temperature reached in the previous cycle, very few A.E. occurred irrespect of the heating rate in the subsequent cycles. We suggest that there is an equilibrium state of the thermally induced cracks, which is a function of temperature but is independent of the heating rate; in the slower experiment, the equilbirium state of crack extension is achieved by slower growth, which is associated with smaller amounts of acoustic emission.

Thermally induced acoustic emission in westerly granite

In acoustic emission studies on granite under uniaxial compression, clear hysteresis of emission numbers related to the stress history (Kaiser effect) was observed below dilatancy onset stress. The maximum stress loaded in the past is memorized in rocks. Above dilatancy, we could not observe such a hysteresis. These features are explained by the nature of irreversible sudden advances of microcrack tips. Recovery process time of this hysteresis is over one month under room humidity and temperature conditions. The results in this paper cast foundation for the applicability of geostress measurements by the acoustic emission method.

Stress memory of crystalline rocks in acoustic emission

Rapid, catastrophic propagation of cracks is described in terms of stress concentration and energy balance conditions from a fracture-mechanics viewpoint. Slow crack propagation by stress corrosion or other mechanisms is also presented in fracture-mechanics terms. Data on slow crack propagation and accompanying acoustic emission in a variety of ceramics and glasses are reviewed. A survey is presented of the application of these results to the quantitative treatment of strength, dependence of strength on loading rate, time to failure, proof testing, acoustic emission monitoring, thermal shock, impact damage, and erosive machining. Some limitations in the application of fracture mechanics to ceramics are discussed.

John B. Wachtman

Papers

Highlights of Progress in the Science of Fracture of Ceramics and Glass

Highlights of progress in the science of fracture of ceramics and glass